CDs and diffraction and cheap precision position detection

[tevarin]

I posted a while ago on using a CD and a flashlight to make a rainbow for Kalina. Here's a cool youtube video on how to measure the spacing of CD grooves using the same principle.

http://www.youtube.com/watch?v=IKKFPtcaZpQ

It occurs to me that CD (or DVD) grooves are a precision manufactured item that nearly everyone in the world can get their hands on. So they would make a good basis for calibrating a self-replicating machine.